The present invention relates to a noise level updating system for reducing a noise component included in an input signal.
Heretofore, a noise reduction system, wherein a signal component having a level lower than a predetermined level is cut as noises, is known.
The present applicant has disclosed a noise reduction system in the Japanese Patent Application Laid Open (Japanese Patent Application 9-310231). In the system, a noise level of a sound portion of an inputted music signal is detected by a noise analysis section, a signal lower than the noise level is cut.
The noise analysis section will be described with reference to FIG. 4. A high frequency component higher than 13 KHz is extracted from an input signal Si by a high pass filter 1 to produce a high pass signal Sh. The high pass signal Sh is converted to a rectified signal Shn by a rectifier circuit 2, and a low frequency region (an envelop component of the rectified signal Shn) is extracted by a low pass filter 3 to produce an extracted signal Sln which is inputted in a level analysis circuit 4.
On the other hand, the input signal Si is also applied to a rectifier circuit 5, thereby producing a rectified signal Sin which is fed to a low pass filter 6 to produce an extracted signal Sl. The extracted signal is fed to the level analysis circuit 4. The level analysis circuit 4 samples signals Sln and Sl and holds a minimum value, and produces the minimum value as a noise level detecting signal Sa based on the extracted signals Sin and Si.
Referring to FIGS. 5a to 5c, FIG. 5a shows a waveform of an entire frequency region of the input music signal Si, and FIG. 5b shows signals Shn (solid line), Sln (dotted line).
FIG. 5c shows a waveform of the signal Sa. As shown in FIG. 5c, the level analysis circuit 4 produces a minimum value at every sampling timing as the signal Sa. Last of all, the minimum value signal Sa is held constant.
However, since the minimum value signal Sa is held constant, noises can not be reduced when the level of the noises heightens.